1. Field of the Invention
The present invention relates to a light beam scanning apparatus for scanning a plurality of laser light beams in an image forming apparatus, such as a digital copying machine or a laser printer, which exposes a single photosensitive drum by simultaneously scanning a plurality of laser light beams over the photosensitive drum, thereby forming a single electrostatic latent image on the photosensitive drum.
2. Description of the Related Art
In these years, various kinds of digital copying machines have been developed, which perform image formation, for example, by scan/exposure using a laser light beam (hereinafter referred to as “light beam”) and an electrophotographic process.
A digital copying machine that uses a multi-beam system has recently been developed. In this digital copying machine, a plurality of light beams are produced to perform simultaneous scan in units of a plurality of lines, thus increasing the speed of image formation.
The digital copying machine using the multi-beam system comprises a plurality of semiconductor laser oscillators for producing light beams; a multifaceted rotary mirror, such as a polygon mirror, which reflects the light beams from the laser oscillators toward the photosensitive drum and causes the light beams to scan the photosensitive drum; and an optical system unit functioning as a light beam scanning apparatus, which includes a collimator lens and an f-θ lens as main components.
In the conventional multi-beam type digital copying machine, in order to form a high-quality image, the optical system unit performs a scan-directional exposure position control of light beams (i.e. light beam position control in a main scan direction) and a light beam passage position control (i.e. light beam position control in a sub-scan direction).
An example of this technique is proposed in Japanese Patent Application No. 11-270894. The proposed technique aims at performing a light beam position control by detecting light beams with high precision over a wide range. A pair of sensors detect a sub-scan-directional passage point of a light beam that scans the surface of the photosensitive drum.
The scan position of the light beam is determined on the basis of a value that is obtained by integrating a difference between outputs of the paired sensors. The integration value indicative of the beam scan position is computed by a processing circuit connected to the paired sensors. The processing circuit comprises a plurality of operational amplifiers, and a difference between electric signals detected by the sensors is integrated by an integrator.
Japanese Patent Application No. 2001-290204 proposes a technique wherein a pair of sensors for sensing light beams comprise two sawtooth-shaped patterns. In this proposed technique, like the technique of Japanese Patent Application No. 11-270894, the beam scan position is determined by a processing circuit that integrates a difference between outputs of the paired sensors.
Further, Japanese Patent Application No. 11-270894 or Japanese Patent Application No. 2001-290204 proposes a technique wherein in a case where an output from a processing circuit that processes output signals from the paired sensors is saturated, the output signals from the paired sensors are corrected, thereby detecting the beam scan position.
There is also a proposed method of efficiently determining and controlling the amount of correction.
However, even if control operations are performed, the controlled beam position may deviate due to a drift or a time-dependent variation of a galvano-mirror, leading to degradation in output image quality.
In addition, when the beam position is detected, a window comparator is used to adjust an instruction range, and measurement is effected by determining whether the beam position is in an upper region, a middle region or a lower region of the range. The method of determining the upper, middle, lower regions is performed by acquiring data units corresponding to the number of polygonal facets of the polygon mirror. A length of time is consumed in setting the width (range) of the window, and exact determination cannot be performed.
Moreover, data set in D/A converters is frequently rewritten due to noise, etc., and exact control cannot be performed. Consequently, an exact control result cannot be output.